It is a common networking issue to plan the network in order to manage the traffic flow, provide quality of service and protection and to allow an efficient use of all the network resources. In the environment of an Ethernet network this ability to plan the available virtual LANs (VLANs) does not currently exist. Ethernet technology requires the use of protocols belonging to the spanning tree protocol (STP) family, in order to make the best of the available network topology. The use of STP, has the effect of reducing the active topology of the network to a tree, also known as spanning tree in the context of STP-related protocols, which excludes all other links in the networks that may be used in case of failure. The STP tree thus does not provide any information relating to available routes in case of failure.
STP has evolved into a different protocol called rapid spanning tree protocol (RSTP) in order to reduce the convergence time in the case of route failures. However RSTP does not improve the efficient use of the available resources in the network because it uses a single tree on the physical topology. In order to overcome the limitations associated with STP and RSTP, a multiple spanning tree protocol (MSTP) has been standardised. MSTP allows configuration of a number of different trees on the physical topology of the network. Each tree (sometimes referred to as an MSTP instance) is independent of the other trees on the network. This means that loops can be prevented by assigning two links or nodes that form a loop on the physical topography of the network to different trees in that network. Similarly, traditional STP or RSTP planning forces the protocol to exclude the use of one of the two links where a loop is formed. It should be noted that MSTP provides for there to be more links within the physical topology of the network plan than would be the case with STP. Concerning the VLANs, an MSTP provides the ability to map a group of VLANs on each MSTP instance. Usually, the number of VLANs that a network operator has to be accommodated is bigger than the number of MSTP instance that can be configured.
One of the critical problems today is that there is no effective method of mapping the VLAN's onto appropriate trees. In actual fact, the mapping of VLAN's onto the trees is generally performed manually during the planning stage of the network. This is a serious limitation to the efficiency of MSTP due to the intensive calculation required to determine the suitable number of trees to be used and the mapping of VLAN's to those number of trees. As this is done manually the design of the paths or routes through the network become increasingly difficult for the operator to perform. In addition, this is only half the problem as the same calculations have to be made for one or more backup paths for each route. Moreover, when primary and backup trees must be calculated according to MSTP rules, the problem is very complex without the support of a procedure. This can significantly increase the amount of mapping the operator must perform.
At the present time Ethernet is a very attractive technical solution due to its simplicity and relatively low-cost technology and the need to find a way of using Ethernet with quality of service support is very real. This is particularly the case in the application of Ethernet technology to metro networks. In this environment quality of service performance and carrier class performance are vital.
At present there are no ways of guaranteeing quality of service, “carrier class” performance and optimised traffic balance in Ethernet based networks. These functions are generally referred to as “traffic engineering”. In actual fact the main way in which quality of service is provided in current environment is by “over provisioning”. However over provisioning presents two main drawbacks: there is no guarantee of quality of service and generally a considerable waste of resources.
MSTP provides the possibility to configure a greater number of trees on the physical topology so as to augment the number of physical links that can be used, thereby improving the network resource utilisation. However, operators continue to manually configure the networks and tend to use fewer trees. As a result Ethernet based networks are generally relatively small and with simple network topologies such as a simple tree or ring that can be easily visualised. Typically if a bigger network is envisaged several smaller Ethernet networks are connected by layer 3 (e.g. the IP layer) or using multi-protocol label switching (MPLS) technologies. While this can provide a larger network it is usually at greater than necessary expense, making Ethernet more expensive than other technologies.
In addition, the evolution towards the conversion model of networks requires that Ethernet technology is adopted into bigger networks having generic topologies. For example, large metro networks or Ethernet radio access networks (RANs) are conceivable where any topology or size could apply.